Satellites, launch vehicles and other spacecrafts are often equipped to support a payload having more than one mission mode. For example, a satellite is carried into space in a transport mode by a launch vehicle for release in a suitable orbit about the earth, allowing the satellite to enter an operational mode. One or more mechanical devices may be used to retain the satellite to the launch vehicle until the release point is reached. The mechanical devices are then controlled to release the satellite for separation from the launch vehicle.
Another application relates to latch devices for use in the space industry to provide a temporary release. A payload of a satellite may require the payload to be non-rigidly mounted ("softly mounted") to the body of the satellite when in the operational mode, while it may be necessary or beneficial to rigidly mount the payload to the satellite body when not in the operational mode. For example, the payload may be an optical space instrument that is used for weather imaging or the like. During an imaging operation, latch devices may release the optical space instrument for support by soft mount devices that allow the instrument to move relative to the satellite body. The optical space instrument is then isolated to some extent from mechanical vibrations of the satellite body, and is therefore in a better performance environment. On the other hand, the optical space instrument is preferably rigidly mounted to the satellite body during launch and during on-orbit events that potentially could damage softly mounted payloads. Accordingly, a spacecraft may be equipped with both soft mount devices and resettable rigid mount latches to accommodate the various modes of payload operations.
Another advantage of a resettable latch for use in the space industry is that the controllable latches facilitate testing procedures on the ground. Prior to launch, a satellite and its optical payloads will undergo numerous tests. The tests may be conducted at various sites, requiring the optical payload to be rigidly mounted to the satellite body during transportation from one site to another. A resettable and controllable latch device may be used to lock the payload in place during transportation and to release the payload for a test procedure.
Conventional controllable latch devices are hydraulically, pneumatically or electromechanically driven. Such devices are well known in various industries, including the space industry. In FIG. 1, a solenoid-driven latch 10 is shown mounted to a platform 12. The latch is shown in solid in a locked condition and is shown in phantom in a release condition. A solenoid 14 raises and lowers a bearing member 16 to pivot an arm 18 about a pivot point 20. In the raised position shown in solid, the arm 18 lowers a pair of pinch wheels 22 and 24 to trap a flange 26 between the pinch wheels. The flange 26 is a portion of a bracket 28 connected to a payload 30. Thus, the payload is fixed in position when the arm 18 is in the raised position and secures the flange 26 between the pinch wheels 22 and 24. The solenoid 14 is selectively retracted to lower the bearing member 16. The arm 18 pivots about the pivot point 20 to the lower position shown in phantom. In response, the pinch wheels pivot upwardly to the position shown in phantom and the flange 26 is released. In this condition, the payload 30 is allowed to move to a limited extent by a soft mount device 32. The soft mount device includes a stationary frame 34 connected to the platform 12 by bolts 36. A payload interface 38 includes a spherical end 40 that is free to rotate within the soft mount device 32 to a limited extent. The maximum deflection of the payload interface may be limited by the dimensions of an opening 42 through which the payload interface passes, or may be limited by some other means not shown.
One concern with the prior art structure shown in FIG. 1 is that a soft mount device 32 that is maintained in a deflected condition for an extended period of time will often take a permanent set in the deflected condition. Another concern is that the latch 10 may be inadvertently back-driven from the locked condition to the release condition shown in phantom. When the latch is in the locked condition, excessive vibrations or a properly directed impact with the payload 30 may cause the flange 26 to pull the upper pinch wheel 24 upwardly with a force that is sufficient to cause the payload to be unintentionally released from the locked condition.
While hydraulically driven, pneumatically driven, and electromechanically driven latch devices are conventionally used, other means for capturing or retaining an object are known. U.S. Pat. Nos. 4,900,078 to Bloch and 5,095,595 to Stella et al. describe capture devices using a shape memory alloy material in space applications.
The Bloch patent describes a gripping device having a pair of jaw bars that are parallel to each other and move toward and away from each other to grasp and release an object. An axially movable actuator bar is connected to the jaw bars by four link members. Each link member is pivotally attached to the actuator bar at one end and pivotally connected to the jaw bars at the opposite end. The pivotal links bring the jaw bars together or further apart depending upon the motion of the actuator bar. The movement of the actuator bar is induced by heating a wire formed of a shape memory alloy, such as a nickel-titanium alloy that contracts when heated. The wire is connected to the actuator bar to move the actuator bar in a given direction when the wire is heated beyond its martensitic critical temperature. When no heat is applied to the wire, a returning mechanism moves the actuator bar in the opposite direction.
Stella et al. describe a latch mechanism that uses a strip of copper-based shape memory alloy as an actuator element. When the actuator element is heated, it bends and pushes a cam member. The cam member rotates a lever to deploy a payload, such as an antenna reflector or solar array. Cooling the actuator element returns it to its normal straight position.
Another actuator that uses the shape memory effect is described in U.S. Pat. No. 4,887,430 to Kroll et al. The patent does not present a latch mechanism, but does describe a pair of shape memory alloy springs on the opposite sides of an actuator member for moving the actuator member in opposite directions. A detent retainer may be used to mechanically hold the actuator member once the desired position is reached.
While the prior art devices that utilize the shape memory effect function well under normal circumstances, these devices share the inadvertent-backdrive concern with electronically driven and hydraulically/pneumatically driven devices. That is, many of these devices are susceptible to the actuator being inadvertently switched from one position to another position by vibration or impact.
What is needed is a latch apparatus that is lightweight, resettable and programmable, with the apparatus preferably being highly resistant to switching between modes as a result of vibrations or impact.